Allen repeatedly asked Sandberg when the social network first found out about the ads — and Sandberg eventually said the company had heard about the ads around the time of the US election.

"We were looking at this certainly not as early as we would have liked to, because we wish we had found it before it ever happened," Sandberg said.

"What we started investigating initially was long-term security threats," Sandberg said. "If you think about 2015, 2016, the threats most people were worried about were hacking, taking down accounts, getting into your email account and sharing all of it."

But she went on to say that Facebook heard rumors around the time of the US election about a potential influence campaign.

Sandberg made the trip to Washington to inform US lawmakers about progress in Facebook's internal investigation into the Russian ads on the site.

Elsewhere in the interview, Sandberg said she "absolutely" supported the full release of the Russian ads that were promoted on Facebook.

"Things happened on our platform in this election that should not have happened, especially troubling foreign interference in a democratic election," Sandberg said. "We know we have a responsibility to do everything we can to prevent this kind of abuse."